In modern wind turbines, one of two systems is used to control the power generated by the turbine generators, especially in situations of excess winds. Both control systems involve a reduction in the lifting force transferred from the wind to the rotor blades of the wind turbine.
By the first method, the pitch system, control is obtained by pitching the blades around the longitudinal axis.
By the second method, the stall system, control is obtained by using a blade profile designed to stall when situations of excess winds occur. In large wind turbines, the stall system may be combined with pitchable blades in order to establish a so-called active stall system.
FIG. 1 shows a prior art pitch system in which the necessary force for pitching the blades is supplied by a hydraulic system. The main part of the hydraulic system is positioned in the nacelle of the wind turbine and provides hydraulic pressure to a number of actuators through a hollow low-speed shaft. The actuators are positioned in the wind turbine hub which connects the blades with the shaft. The actuators transform the hydraulic pressure into mechanical force used when pitching the blades.
With the prior art control systems comprising hydraulics, a number of problems have come into view. In particular, the arrival of large, megawatt wind turbines has exposed problems with the hydraulic systems and the transmission of hydraulic pressure from stationary to rotating components of the hydraulic systems in the wind turbines. Also, the necessary hallow shafts are not suitable or useable in connection with large wind turbines and are generally too expensive in production. The distance between the different hydraulic components may result in significant pressure capacity loss and a knocking noise affecting the lifetime of the hydraulic components.